American troops are scheduled to withdraw from Iraq by the end of 2011 --but don't bet that all of them will leave.

There are several reasons to maintain a residual, combat-capable U.S. military presence in Iraq, the most obvious being the proxy war on Iraq waged by Iran's tyrannical regime.

That proxy war has gone on since 2003, but within the last year, as U.S. forces have withdrawn, Iranian troublemaking has increased.

The Obama administration has noticed. This week, U.S. Secretary of Defense Leon Panetta told a group of American soldiers in Iraq that Washington is "very concerned about Iran and the weapons they're providing to extremists in Iraq."

Panetta could have added, with certainty, that Iran provides Iraqi gangs, extremist militias, and al-Qaida remnants with money and political support. Iranian intelligence services and special forces may also be providing some of these groups with operational planning and targeting advice.

Iran's radical Islamic regime knows an Iraqi democracy on its western border threatens its very existence. Iran's mullahs fear Iraq's democracy because it gives the Iranian opposition Green Movement an authentic Middle Eastern model for democratic political action. Waging a proxy war on Iraq serves the mullahs' domestic political goal of repressing their own population.

Credit Panetta with being an Obama administration official who will publicly raise the prospect of keeping residual U.S. forces in Iraq. At the moment, Washington and Baghdad are engaged in a complex diplomatic tap dance. Both governments acknowledge the need to remain allies. Washington wants the Iraqi government to extend an invitation to keep U.S. military forces in Iraq, but the Iraqis are reluctant. Though the nation's two most prominent political leaders, Prime Minister Nouri al-Maliki and former interim Prime Minister Iyad Allawi, continue their feud, there are indications both men think a U.S. military presence will help deter Iran. Radical Shia cleric Muqtada al-Sadr complicates Iraqi domestic politics. He wants the U.S. out right now. Sadr, however, is a violent bully who is little more than Iran's mouthpiece.

The Obama administration's mixed messages have also added to the diplomatic muddle. Candidate Barack Obama vowed to quit Iraq. He also promised Iran's tyrants unconditional negotiations. Burned by Iranian belligerence and hypocrisy, President Obama has slowly discovered that bugging out of Iraq isn't such a good idea.

Late last year, the U.S. government acknowledged that the Iraqi Army and Air Force will not reach what the Pentagon calls Minimum Essential Capability (MEC) by December 2011. MEC is jargon for being able to secure the country from internal attacks (by gangs and terrorists), as well as defend Iraqi airspace and territory from conventional attack.

The Iraqi Army has improved. The April 2008 Charge of the Knights offensive demonstrated that the Army's best units were able to plan and execute a multidimensional internal security operation. The quality of individual units even within Iraq's Quick Intervention Corps varies widely, however.

The U.S. will continue to provide training assistance, logistical support and intelligence support to the Iraqi Army. Realists in Baghdad and Washington, however, know that a reinforced U.S. division would provide a reliable, readily available backup in an emergency. The Iraqi Air Force exists, barely. After some two years of indecision, the IAF is on the verge of buying a squadron of U.S.-made F-16 jet fighters. It intends to add a second squadron (for a total of 36 aircraft) as funds become available.

The F-16 is the plane Iraq needs. Two F-16 squadrons, supported by an integrated surface-to-air missile defense system, would go a long way toward securing Iraqi airspace -- if the pilots are well-trained and the planes are well-maintained.

Building those two squadrons and training their personnel takes time, however. 2016 is the earliest date the IAF could go it alone. Who defends Iraqi airspace in the interim? Air defense arrangements with Kuwait, Jordan and Turkey are a possibility, but here's the realistic answer: the U.S. Air Force.